The present invention relates to production and recovery of fine valve metal powders, and, in particular, fine tantalum powders, and products incorporating the powders.
Tantalum anodes, made from tantalum powder, have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. Capacitors with tantalum anodes typically are manufactured by compressing tantalum powder to form a pellet, sintering the pellet in a furnace to form a porous tantalum body (electrode), and then subjecting the porous body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body. Development of powders suitable for making tantalum capacitors has resulted from efforts by both capacitor producers and tantalum processors to delineate the characteristics required for tantalum powder for it to best serve in the production of quality capacitors. Such characteristics include specific surface area, purity, shrinkage, pressability, and the like.
Various techniques have been practiced or disclosed for the production of tantalum powders by a reduction of tantalum that can affect one or more of these tantalum particle characteristics. Typical techniques, such as those briefly outlined in Cabot Corporation's U.S. Pat. No. 5,234,491, are reviewed below.
Tantalum powder has been produced in a chemical method by adding sodium to K2TaF7, which has been previously dissolved in molten salt. In this method, the K2TaF7 or other reducible tantalum halide and diluent salts are heated in a reaction vessel to a temperature above the melting point of the salt mixture. Liquid sodium is then added. The bath is held at essentially isothermal conditions, with stirring of the bath effected by an internal agitator. The resulting powder has a wide range of particle sizes. In order for these materials to be acceptable for the manufacture of anodes for electrolytic capacitors, they may require extensive classification to obtain the desired particle size distributions. The capacitive charge that can be obtained from anodes derived from these powders typically is in the intermediate range. A modification of this stirred liquid phase reaction scheme involves the introduction of diluent salts to the stirred reaction bath. The addition of diluents such as NaCl and KCl to the K2TaF7 allows the use of lower bath temperatures. However, this modified process results in agglomerates of finely divided material, a tendency to pick-up impurities, and production of excessive fines.
In another method, solid diluent salt and K2TaF7 are mulled with liquid sodium and the mixture is heated to the point of initiating a spontaneous exothermic reaction. This exothermic reaction is not easily controlled and, therefore, the product characteristics include varying particle sizes, broad particle size distributions, and varying electrical characteristics. These materials require classification to remove fine and coarse particles from the finished product prior to their utilization in the manufacture of anodes for electrolytic capacitors.
Potassium fluorotantalate (K2TaF7) also can be electrolytically reduced to tantalum in a molten bath with diluent chloride and fluoride salts of sodium and potassium. In addition, tantalum powder can be made by exothermic reaction in a closed vessel wherein the K2TaF7 is arranged in alternate layers with reducing agent.
U.S. Pat. No. 4,149,876 discloses techniques for controlling particle size of tantalum powder product in a reduction process wherein molten sodium is added to a molten bath of K2TaF7 and a diluent salt wherein the sodium addition is controlled during a nucleation period. This patent defines the period of the overall reaction during which the temperature of the charge increases from the initial bath temperature to the reduction temperature as the “nucleation period.” When it was desired to produce very fine particle size tantalum powder to be used in the manufacture of anodes employed in the manufacture of high capacitive charge electrolytic capacitors, the sodium metal is disclosed as being added at a very high rate until the reduction temperature is reached. It was also reported in this patent that the rate of sodium injection (feed rate into the reactor) during the nucleation period has an inverse effect on the particle size of the finished product. More specifically, the patent also teaches that the average size of the finished product was inversely related to the rate of temperature rise with respect to time during the nucleation period, and to the time to complete the addition of the required stoichoimetric amount of sodium at that specified reduction temperature, called the “growth period”.
U.S. Pat. No. 4,684,399 discloses a process for producing tantalum powder wherein a tantalum compound is added in a continuous or incremental manner to a reactor during the course of the reaction with a reducing metal. The rate of continuous addition or the amount of each increment can be varied depending on the particular tantalum powder product characteristics desired. Continuous addition or the addition of smaller increments tends to favor increased capacitance. The addition of the reducing agent as a single unitary charge prior to the introduction of the tantalum compound, or alternatively, in a continuous or semi-continuous manner is also disclosed.
Canadian published patent application No. 2,622,336 discloses a method for producing a valve metal comprising the step of melting, in a first vessel, a mixture including a valve metal precursor and a diluent, and transferring the mixture to a second vessel in order to mix it under the same or different conditions of temperature and residence time, during which the reaction of the valve metal precursor to form a valve metal is initiated. The ratio of diluting salt to valve metal precursor is generally greater than 1:5 and mostly greater than 1:20. The fineness of a particle is based on reaction temperature, a reducing agent ratio, and the rate of molten salt dilution.
WO 2007/130483 A2 discloses a method of forming fine tantalum particles by dispersedly adding at least one reducing agent into potassium tantalum fluoride dissolved in molten salt, and reducing surface roughness and/or increasing neck thickness of the fine tantalum particles by subjecting the fine tantalum particles to an electrolytic and electrodeposition treatment. The reaction for forming the tantalum particles comprises use of a very high rate of dilution after dissolving K2TaF7 in potassium chloride, while carrying out incremental addition of sodium and nitrogen addition. Fine particles are collected, without carrying out continuous additions.
Japanese unexamined patent application publication No. 2006-002241 discloses in a first embodiment a process and manufacturing installation for manufacturing a valve metal by reduction of K2TaF7 with addition of sodium in a diluent containing potassium fluoride and potassium chloride, wherein metallic tantalum and then some diluents are removed from the reactor with a valved exhaust pipe at the bottom of the reactor. The remaining diluent in the reactor is replenished with potassium chloride and potassium fluoride to replace the removed diluent amounts, and is reused. In a second disclosed embodiment, after the reaction step, some upper diluents are removed with an exhaust pipe for disposal and the remainder of diluent is fed to another reactor for reuse, and the metallic tantalum is collected by opening the reactor after cooling. The diluent remainder is replenished with potassium chloride and potassium fluoride to replace the removed diluent amounts, and is reused.
Japanese unexamined patent application publication No. 2006-546787 discloses a method for making tantalum with introduction of heated nitrogen gas into tantalum fluoridation potassium and potassium chloride in a reactor with dilution made to about 15 to 25 times, and incremental sodium is added about 40-60 times after the K2TaF7 dissolution.
The present inventors have realized that it would be advantageous to provide a reaction system that allows for the production and isolation of fine tantalum particles or other valve metal particles from the reaction system in a more efficient and improved manner.